The present invention relates to a sealing cover for an electronic package and to a method of making a cover with integral solder and plated sealing surfaces. More particularly, the invention relates to a sealing cover for use with a hermetically sealed package that contains a semiconductor device, a surface acoustical wave device, oscillator or other microelectronic component.
Typically, metal lids used as covers for hermetically sealing electronic packages have employed a plated metal lid, such as a lid with iron-nickel alloy substrate over which nickel and gold layers are plated, and using a solder preform, generally attached by tack welding before sealing. The present invention provides an alternative method of making a sealing cover more economically by combining a series of process steps in a unique combination to produce a novel cover with integral solder and plated sealing surfaces.
The industry-standard sealing cover is a metal cover with an iron-based substrate material, typically Alloy 42, or Kovar, which has been punched or stamped and then plated with nickel as a corrosion barrier and gold as a solderable metal layer. To this plated stamping, a gold and 20% tin eutectic solder preform is attached, usually in the form of a "ring" appropriately sized to fit the stamping and the package to which the stamping will act as a cover. The ring is secured to the package initially by electric spot welding. The assembly process described is quite labor intensive and requires careful control. Moreover, the assembled parts are relatively fragile and careful handling and packaging is necessary to protect the integrity of the assembly. For large metal covers, i.e., 0.5 inches or greater in a major dimension, the primary costs of manufacture is in the precious metals involved, while the manufacturing and assembling costs are comparatively less. However, as the size of the sealing cover to be made decreases, assembling and manufacturing costs become an increasing cost factor, and for small metal covers, i.e., 0.3 inches or less in a major dimension, the assembly labor costs may exceed the material costs.
To reduce manufacturing and labor costs associated with small sealing covers, roll cladding has been proposed to attach the corrosion barrier, solderable material and solder material to the substrate. After assembling the roll clad composite in strip form, segments are stamped or punched from the strip to form sealing covers. However, products made by this method have several significant drawbacks. The edges of the cover after stamping have exposed substrate material which is both subject to corrosion and does not provide a solderable surface on which a solder fillet can readily form when the cover is sealed to the package. The solder fillet is known to be an important factor in the integrity of the finished package, especially in hermetic sealing applications, by preserving the seal. Furthermore, multi-component cladding processes are also difficult to control due to the complexity of bringing five layers of material together in proper alignment, with correct incident angles to the mill rolls and proper tension to achieve clad adhesion and yet not stretch the material, while constantly maintaining cleanliness of the individual layers. To achieve the desired ultimate thickness in such a process, each layer of material must first be rolled to precise thickness tolerances, slit to correct widths and cleaned; all of which involve significant manufacturing costs. The gold layer typically used to assure adherence of the solder layer and the multi-component cladded composite also has to be considerably thicker than functionally required since extremely thin gold layers, e.g., foils, are too fragile to handle with the back-tension required for the roll cladding process. The need to use excess gold also adds to the material costs.